FXAlg #902: LFO Sweep Filter
Algorithm Reference-120
FXAlg #902: LFO Sweep Filter
LFO-following stereo 2-pole resonant filter
Allocation Units:
2
The LFO following filter is a stereo resonant filter with the resonant frequency controlled by an LFO (low-frequency
oscillator). The filter type is selectable and may be one of low pass (i), high pass (ii), band pass (iii), or notch (iv). See
the section of this book on FXAlg #900 for diagrams of the filter actions.
The resonant frequency of the filter will sweep between the minimum frequency (Min Freq) and the maximum
frequency (Max Freq). The minimum and maximum frequencies may be set to any combination of frequencies
between 16 and 8372 Hz. Note that the terms minimum and maximum frequency are a reference to the resonant
frequencies at the minimum and maximum envelope levels; you may set either of the frequencies to be larger than
the other, though doing so will just invert the direction of the LFO. Meters are provided to show the current
resonance frequencies of the left and right channel filters.
The filter Resonance level may be adjusted. The resonance is expressed in decibels (dB) of gain at the resonant
frequency. Since 50 dB of gain is available, you will have to be careful with your gain stages to avoid clipping.
You can set the frequency of the LFO using the LFO Tempo and LFO Period controls. You can explicitly set the
tempo or use the system tempo from the sequencer (or MIDI clock). The LFO Period control sets the period of the
LFO (the time for one complete oscillation) in terms of the number of tempo beats per LFO period.
The LFO may be configured to one of a variety of wave shapes. Available shapes are Sine, Saw+, Saw-, Pulse and
Tri. Sine is simply a sinusoid waveform. Tri produces a triangular waveform, and Pulse produces a series of square
pulses where the pulse width can be adjusted with the ÒLFO PlsWidÓ parameter. When pulse width is 50%, the
signal is a square wave. The ÒLFO PlsWidÓ parameter is only active when the Pulse waveform is selected. Saw+ and
Saw- produce rising and falling sawtooth waveforms. The Pulse and Saw waveforms have abrupt, discontinuous
changes in amplitude which can be smoothed. The pulse wave is implemented as a hard clipped sine wave, and, at
50% width, it turns into a sine wave when set to 100% smoothing. The sudden change in amplitude of the sawtooths
develops a more gradual slope with smoothing, ending up as triangle waves when set to 100% smoothing.
Configurable Wave Shapes
Parameters:
PAGE 1
Wet/Dry
0 to 100%wet
Out Gain
Off, -79.0 to 24.0 dB
LFO Tempo
System, 1 to 255 BPM LFO Shape
Sine
LFO Period
1/24 to 32 bts
LFO PlsWid
0 to 100%
LFO Smooth
0 to 100%
Sine
Saw+
Saw-
Pulse
Tri
PulseWidth
